There are many known power assist steering systems that use hydraulic power, electric power or a combination thereof. Examples of electric power assist steering systems are shown in U.S. Pat. Nos. 3,983,953 and 4,415,054.
In U.S. Pat. No. 3,983,953, an electric assist motor is coupled to an input steering shaft and energized in response to torque applied to the vehicle steering wheel. When energized, the electric motor applies a rotary drive force to a pinion gear of a rack and pinion gear set and thereby assists the driver in accomplishing the steering maneuver.
U.S. Pat. No. 4,415,054 describes a system in which an electric assist motor having a rotatable armature is coupled to a nut of a ball-nut assembly. A screw of the ball-nut assembly is integral with a rack member of a rack and pinion gear set. Rotation of the armature produces linear movement of the rack member. Linear movement of the rack member effects steering movement of the steerable wheels of the vehicle. The electric assist motor is actuated in response to operator-applied torque to the vehicle steering wheel. The electric assist motor, when actuated, applies a linear drive force to the rack member to assist the driver.
Previous electric assist steering systems have typically utilized four solid-state switching devices connected in an "H" bridge arrangement to the positive and negative terminals of the vehicle battery and to the electric assist motor. The solid-state switching devices are controlled by an electronic control unit ("ECU") in response to electrically sensed steering direction and sensed steering torque applied to the vehicle steering wheel. The ECU controls the direction of current flow through the electric assist motor by controlling actuation of the solid-state switching devices in pairs. The pair of switching devices actuated depends upon the sensed direction of the applied steering torque. The ECU controls the magnitude of current flow through the electric assist motor by pulse width modulating the signal controlling actuation of at least one of the solid-state switching devices of a selected pair. The magnitude of the power assist is proportional to motor current which, in turn, is controlled by controlling the duty cycle of the pulse width modulated actuation signal.
Such steering systems have proven effective in achieving the desired function of providing adequate steering assist under all vehicle operating conditions. However, it is desireable to reduce the cost of an electric assist system as compared to known systems. A significant fraction of the system cost is represented by (i) the component cost for electronic devices capable of switching large amounts of current, including peripheral costs such as heat sinks, etc., and (ii) the large number of electrical components needed to provide adequate control of the electric assist motor.
During a high demand steering maneuver, such as dry surface parking, an electric assist motor requires as much as 65 amps to provide adequate assist. Solid-state switching devices capable of switching 65 amps are relatively expensive. Four solid-state switching devices are required in a typical H-bridge network. Also, a large number of other electrical components are necessary to control the H-bridge in response to sensed steering torque and sensed steering direction.
One design, shown in U.S. Pat. No. 4,598,787 to Drutchas and assigned to the assignee of the present application, is directed to an electrical assist steering system that has a reduced number of electrical elements. In the steering system disclosed in the '787 patent, a mechanical switch arrangement is provided including four mechanical switches connected in an "H" network to an electric assist motor, to one terminal of the vehicle battery, and to a drive circuit. The drive circuit includes a single solid-state switching device. Upon application of torque to the vehicle steering wheel, a pair of the four switches are mechanically closed across the electric assist motor to control the direction of electrical power applied thereto. This arrangement thus does not require electrical elements to sense direction or electrical elements to control closing of the pair of switching devices. The magnitude of the electrical current applied to the electric assist motor is controlled by pulse width modulating the single solid-state switching device in the drive circuit.
Even though the arrangement disclosed in the '787 patent to Drutchas significantly reduces the number of electrical elements required for an electric assist steering system, the solid-state switching device that controls motor current must still be capable of switching 65 amps, and must do so at the frequency of the pulse width modulated control signal.
It is desirable to provide a more cost effective electric assist steering system than has heretofore been provided that (i) utilizes a minimum number of electrical elements while maintaining full system control and (ii) provides sufficient operating current to the electric assist motor under all operating conditions.